Instead of writing file@name="x" one can incorporate name of file in namespace of OpenPGP notation itself as filename@torproject.org.

I think it's best if OpenPGP notations follow existing conventions. For example, issuer-fpr@notations.openpgp.fifthhorseman.net is one of the more common ones. Notations ​might even be standardized one day. Now, for file name there isn't a convention yet, but I think filename@torproject.org isn't a good idea, because it's difficult to parse with general purpose gpg verification tools. (Both keywords, filename and homepage are variable.) Ideally, this becomes a common convention and perhaps even one day gpg [or wrappers] start using it.